Adults who lose their hearing later in life also are more likely to have a hard time concentrating on a book or remembering a simple conversation, Johns Hopkins research has found.

The same brain functions that affect hearing also may cause problems with memory and other cognitive function, according to the study, published this month in JAMA Internal Medicine.

It is the latest to support a link between hearing loss and decline of memory. The Hopkins researchers said that many people view hearing loss as an inconvenience of old age but that it may also contribute to more serious health problems.

The scientists used brain tests to study patients with hearing loss over six years and found that their cognitive abilities declined 30 percent to 40 percent more than in those with normal hearing. People who lost their hearing also developed problems remembering and thinking at least three years sooner than others.

The researchers aren't sure yet why brain function is inhibited with hearing loss but are looking at several theories.

Social isolation could play a role, said the lead researcher, Frank Lin, an assistant professor at the Johns Hopkins University School of Medicine and the university's Bloomberg School of Public Health. People who can't hear may not be as engaged in conversations because it takes too much effort and they don't want to embarrass themselves.

"You're less likely to go out because you can't engage with people," Lin said. "You stay home alone and the problem doesn't get better."

The brain also may compromise memory and thinking to focus on hearing in people with hearing loss. When a person loses hearing, it is because the inner ear is no longer encoding sounds with "good fidelity," Lin said. "Your brain is constantly getting a garbled signal. You're now engaged in effortful listening. You have to expend more energy to understand what is being said."

The researchers also said that there may be underlying brain damage that leads to both hearing and cognitive losses.

Lin, who has studied the effects of hearing loss on older people for many years, including its link to dementia, estimates that as many as 27 million Americans over age 50 suffer from a form of hearing loss. Two-thirds are age 70 or older.

His study is believed to be the first to look at the effects of hearing loss on brain function over time.

He looked at 1,984 older adults who participated in the Health ABC Study, a long-term study in Pittsburgh and Memphis that monitored the health of the elderly. None of the participants showed signs of cognitive decline when the research began in 2001 and 2002 with memory and hearing tests. Hearing loss is defined as only recognizing sounds louder than 25 decibels, or the sound of a whisper.

Participants in the study were given brain tests that asked them to memorize words and answer questions about years and dates. They were also tested on how long it took them to match numbers and symbols.

Lin plans to conduct further studies to determine what is causing the memory decline and to see if the use of hearing aids can help maintain memory function.

Previous studies by Lin have shown that hearing loss is widely undertreated and that only one in seven people with hearing loss use hearing aids. Most people ignore the problem or learn to live with it. Because people are living longer, there are more people with hearing loss, he said.

A 2011 study showed that the risk of hearing loss doubles in a person every decade. Hearing loss is more common in those with hypertension and diabetes, and more common among white men than in women and African-Americans, that study found.

"Ultimately, the big question is, if you treat hearing loss and treat it well, can you reduce cognitive decline and the risk of dementia?" Lin said.

Lin said doctors need to pay closer attention to patients who may have hearing problems and encourage them to seek treatment.